This invention relates to a measuring device for measuring the potential of an underground metallic structure, which is cathodically protected by means of an externally applied DC voltage, relative to the earth or soil in which it is situated.
Cathodic protection is used as one method for combating corrosion of metallic structures, such as pipelines, thanks, cables, piers, and the like, which are at least partially buried in the earth. Cathodic protection is accomplished by raising the potential of the surrounding earth by means of introducing a direct current into the earth at a sufficient potential to protect the metal surface. This can be achieved by burying electrodes in the earth at the required distance from the structure in order to obtain a proper current distribution to the metal surfaces to be protected. Cathodic protection is applicable to structures made from all the usual metallic materials of construction such as steel, aluminum, copper, zinc and alloys thereof. As an example, it may be mentioned that, depending on the type of earth in which a cathodically protected structure made of ordinary steel is situated, the potential of the structure, measured relative to a Cu/CuSO4 reference electrode placed in the soil must be -850 mV to -950 mV or lower in order to effectively prevent corrosion.
To insure the continuing proper operation of such a cathodic protection system, this potential should be checked regularly by means of measurement against a reference electrode. However, direct measurement of the voltage difference between the protected structure and the reference electrode is not always reliable, due to the voltage drop in the earth caused by the passage either of the applied protective current or of possible stray currents derived from nearby electrical installations, and the correct potential of the structure with respect to the earth would not then be measured.
The reliability of the potential measurement can be improved by using a measuring electrode electrically connected to the structure, and positioning the measuring electrode and reference electrode with respect to one another in such a way that no extra voltage difference is produced between them by the passage of extraneous currents through the soil. This can be achieved by a measuring rod provided with both a measuring electrode and a reference electrode, and a measuring circuit to measure the potential difference between the measuring electrode and the reference electrode. The measuring rod is adapted to be driven into the soil near the cathodically protected structure, and the measuring electrode is electrically connected to the protected metallic structure.
Such a device for the measurement of the protection potential is known from German Patent Application No. 2,241,648 (laid open for public inspection). In the measuring rod there described, a measuring electrode (in this case a sintered nickel disk impregnated with mercury) and a Cu/CuSo.sub.4 reference electrode are placed in close proximity, but electrically insulated from one another, in a protective steel tube which is provided with a sharp tip so that it can be driven into the soil. Openings are provided in the wall of the tube near the electrodes so that the electrodes can come into contact with the surrounding soil.
This known device, however, has the disadvantage that because the contact surfaces where the soil meets the electrodes are situated inside the protective tube, the good contact between the soil and the electrodes necessary for a reliable measurement is not always obtained, so that the measurements may be unreliable. Furthermore, the openings in the wall of the tube must be carefully cleaned after each time the measuring rod is used, which is inconvenient. Moreover, the construction of this known measuring rod is relatively complicated and hence liable to malfunction, for instance, the electrode can slip out of place in the tube without this being noticed immediately.